Turbine flow meters are a very popular method of electronically measuring liquid flow. Inside such a meter, the flowing liquid in a conduit engages a rotor having a plurality of vanes causing it to rotate at a velocity which is directly proportional to the flow rate of said liquid. As the rotor rotates, a voltage is induced within a magnetic coil or a pulse is generated from an electronic sensor, both mounted outside the conduit each time a vane passes thereby creating an electronic pulse. Each pulse represents a distinct volume of liquid which passes between adjoining vanes.
As best seen in FIG. 1, when dispensing any liquid through a pipe 16 using a turbine meter 10, the usual procedure is to collect the electronic pulses and count, either up or down, until a desired quantity of liquid has passed through said turbine meter 10. At this time a microprocessor 12 in electronic communication with turbine meter 10 sends a signal to close a solenoid valve 14 ending the dispensing process. FIG. 1 shows this process schematically. Since the meter or system electronics are controlling the process the exact timing of opening and closing of solenoid valve 14 is included in this process, an accurate measurement of the liquid dispensed is easily attained.
However, when seeking a total quantity of liquid which passes through turbine meter 10 during a process in which the system is not controlling the flow rate or start and stop functions, turbine meter 10 must be able to respond almost instantly to the change in flow rate or starting or stopping of the flow without any anticipation as to when these events may occur. An example of an operation where the total quantity is sought is the emptying of one tank into another tank. The metering system does not control the process and must simply respond to the start of the operation, changing rates during the transfer, and the termination of flow when the supply tank is empty without any signals as to when these events may occur.
Another common example of an operation wherein the total amount of liquid passed through turbine meter 10 is the filling of a car's gasoline tank at a fueling station. When filling the tank of a car, the operation is manually controlled by the customer. The customer squeezes the handle of a fill nozzle 18 at his discretion, controls the flow rate by how far fill nozzle 18 is opened and shuts off fill nozzle 18 at his discretion or when the tank is full. Many customers “snap” the fill nozzle 18 several times in order to “top off” the tank, usually only adding a very small additional quantity of gasoline to the tank. Turbine meter 10 must be capable of responding almost instantly to these events in order to accurately compute the quantity of fuel transferred. FIG. 2 shows a schematic of such a typical configuration.
In turbine meter 10, prior to initiation of the flow process, the system is at rest. There is no axial (down the pipe) movement of the liquid. The one or more turbine rotors 20 are at rest, and no pulse output from turbine meter 10 is being sent to microprocessor 12. Now as control valve 14 is opened, the axial flow commences in pipe 16 and turbine rotors 20 begin to rotate in proportion to the rate of flow in the pipe. As turbine rotors 20 rotate, the fluid contained within turbine meter 10 also begins to rotate. Thus the fluid within and exiting from turbine rotors 20 of turbine meter 10 have both an axial component and a tangential or rotating component of velocity.
During flow acceleration, from zero to high flow rate, or from low to high flow rate, turbine rotors 20 and turbine meter 10 will respond almost instantly to changes in rate. The low inertia of the turbine rotors 20 compared to high driving momentum of the fluid cause turbine rotors 20 to respond within a few milliseconds to the change of flow rate. Thus an accurate representation of the instantaneous flow rate and an accurate total accumulated quantity of liquid passing through turbine meter 10 can be computed. At steady state flow rates, turbine meter 10 can also provide an accurate representation of the instantaneous flow rate and an accurate total accumulated quantity of liquid passing through turbine meter 10 can be computed.
When control valve 14 is slowly closed or controlled from one more open position to a lesser open condition, i.e.: going from a high flow rate to a lower flow rate, the inertia of turbine rotors 20, and hence a frequency of pulses 24 therefrom and the rotating fluid contained therewithin will accurately follow the change in rate as shown graphically in FIG. 3. The rate of change of flow rate 22 is less than the momentum of turbine rotors 20 and rotating fluid therewithin. During this condition, turbine meter 10 provides an accurate representation of the instantaneous flow rate and an accurate total accumulated quantity of liquid passing through turbine meter 10 can be computed.
When control valve 14 is instantly traversed from a wide open position to a nearly closed position, but flow rate 22 is not completely stopped, frequency 24 of turbine meter 10 responds accordingly. Frequency 24 of turbine rotor(s) 20 and rotating fluid contained within them is only slightly lagging the true flow rate 22 of the liquid. This occurrence happens over a few millisecond period and the error in the computation of instantaneous flow rate and the accurate total accumulated quantity of liquid passing through the meter is extremely small and within acceptable limits. FIG. 3 shows the typical frequency 24 decay of turbine rotors 20 and the “true” rate of change of flow rate 22 of the liquid.
However, when control valve 14 is instantly traversed from an open position to the fully closed position, completely stopping flow rate 22, the momentum of turbine rotors 20 and the fluid contained therewithin cause said turbine rotor to continue to spin in a decaying manner for a finite period of time. When control valve 14 is closed, the axial component of the velocity of the liquid in pipe 16 stops. However the rotating or tangential component of the velocity within turbine meter 10 results in turbine rotors 20 continuing to rotate until the viscous friction within the fluid causes the motion to stop. This condition is known as overspin. The decay rate is a logarithmic function and can take several seconds to completely stop. During this period, turbine meter 10 continues to send frequency pulses 24 to microprocessor 12 which indicates is treated as a continuing flow which results in a “false” increase in the total accumulated quantity passing through the meter. However, for an accurate measurement, these later frequency 24 pulses must not be included in a calculation of the rate or accumulated quantity as they do not represent an axial component or “true” flow or quantity passing through turbine meter 10. While this error may be small relative to the total quantity of liquid, it is desirous to eliminate them from the computations.
Presently, users of turbine meters 10 will often instrument the valve with a switch or otherwise provide a signal to microprocessor 12 that control valve 14 is closed and thus any pulses generated by turbine meter 10 can be ignored. One solution of the prior art is illustrated in FIG. 4 which uses a microswitch 40 to provide said signal. When it is impractical to instrument the control valve, a flow switch 50 can be added to the system, as shown in FIG. 5, to provide a signal to microprocessor 12 to accomplish the same end.
U.S. Pat. No. 6,651,517 entitled “Fuel Dispensing System” which issued on Nov. 23, 2003 to Paul D. Olivier, the present inventor, describes one such solution to this problem. However, the instrumentation of the valve or the addition of a flow switch are expensive solutions to the problem. The flow switch has proven to a problematic solution. The reliability of the flow switch is significantly less than that of the turbine meter thereby diminishing the system reliability while increasing the system cost.
Thus, there is a need for an alternate method to accurately measure the flow rate when going from an open to a fully closed valve.
The present invention meets this need.